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<a name="Loop_002dSpecific-Pragmas"></a>
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<a name="Loop_002dSpecific-Pragmas-1"></a>
<h4 class="subsection">6.62.16 Loop-Specific Pragmas</h4>

<dl compact="compact">
<dd><a name="index-pragma-GCC-ivdep"></a>
</dd>
<dt><code>#pragma GCC ivdep</code></dt>
<dd>
<p>With this pragma, the programmer asserts that there are no loop-carried
dependencies which would prevent consecutive iterations of
the following loop from executing concurrently with SIMD
(single instruction multiple data) instructions.
</p>
<p>For example, the compiler can only unconditionally vectorize the following
loop with the pragma:
</p>
<div class="smallexample">
<pre class="smallexample">void foo (int n, int *a, int *b, int *c)
{
  int i, j;
#pragma GCC ivdep
  for (i = 0; i &lt; n; ++i)
    a[i] = b[i] + c[i];
}
</pre></div>

<p>In this example, using the <code>restrict</code> qualifier had the same
effect. In the following example, that would not be possible. Assume
<em>k &lt; -m</em> or <em>k &gt;= m</em>. Only with the pragma, the compiler knows
that it can unconditionally vectorize the following loop:
</p>
<div class="smallexample">
<pre class="smallexample">void ignore_vec_dep (int *a, int k, int c, int m)
{
#pragma GCC ivdep
  for (int i = 0; i &lt; m; i++)
    a[i] = a[i + k] * c;
}
</pre></div>

<a name="index-pragma-GCC-unroll-n"></a>
</dd>
<dt><code>#pragma GCC unroll <var>n</var></code></dt>
<dd>
<p>You can use this pragma to control how many times a loop should be unrolled.
It must be placed immediately before a <code>for</code>, <code>while</code> or <code>do</code>
loop or a <code>#pragma GCC ivdep</code>, and applies only to the loop that follows.
<var>n</var> is an integer constant expression specifying the unrolling factor.
The values of <em>0</em> and <em>1</em> block any unrolling of the loop.
</p>
</dd>
</dl>




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